1. Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus including an image bearing member and a transfer device contacting the image bearing member to form a transfer nip therebetween, and a toner image formed on the image bearing member is transferred onto a recording medium fed to the transfer nip.
2. Description of the Related Art
In known electrophotographic image forming apparatuses, an unfixed image, known as a toner image, is formed on an image bearing member, i.e., a photosensitive drum. An intermediate transfer member, i.e., an intermediate transfer belt serving also as an image bearing member, contacts the photosensitive drum to form a so-called primary transfer nip therebetween. In the primary transfer nip, the toner image on the photosensitive drum is primarily transferred onto the intermediate transfer belt.
A secondary transfer roller serving as a transfer device contacts the intermediate transfer belt to form a so-called secondary transfer nip. An opposed roller is disposed inside the loop formed by the intermediate transfer belt, facing the secondary transfer roller with the intermediate transfer belt interposed therebetween. The opposed roller disposed inside the loop of the intermediate transfer belt is grounded; whereas, the secondary transfer roller disposed outside the loop is supplied with a secondary transfer bias (voltage). In this configuration, a secondary transfer electric field is formed in a secondary transfer nip between the opposed roller and the secondary transfer roller. The secondary transfer electric field causes the toner image to move from the opposed roller side to the secondary transfer roller side.
A recording medium is fed to the secondary transfer nip in appropriate timing such that the recording medium is aligned with the toner image formed on the intermediate transfer belt. Due to the secondary transfer electric field and a nip pressure applied to the secondary transfer nip, the toner image on the intermediate transfer belt is transferred secondarily onto the recording medium.
In such a configuration described above, when using a recording medium having a coarse surface such as Japanese paper (also known as Washi), a pattern of light and dark according to the surface condition of the recording medium appears in an output image because toner does not transfer well to such an embossed surface, in particular, recessed portions of the surface. As a result, the image density at the recessed portions is lower than the image density at projecting portions. This inadequate transfer of the toner appears as the pattern of light and dark patches in the resulting output image.
In view of the above, in one approach, not only a direct current (DC) voltage, but also a DC voltage superimposed on an alternating current (AC) voltage is applied as a secondary transfer bias, thereby preventing the pattern of light and dark, as compared with supplying only the direct current voltage.
According to experiments performed by the present inventors, the present inventors have recognized that application of the secondary transfer bias as described above causes easily white spots (absence of toner) in an image on the recessed portions of the recording medium. This phenomenon also depends on the degree of roughness of the recording medium, temperature and humidity, electrical resistance at the transfer nip, and so forth.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good transferability regardless of surface conditions of recording media.